feat(vault): vault storage (user-isolated, multi-account)

src/screens/VaultScreen.tsx

@@ -258,7 +258,7 @@ export default function VaultScreen() {
     setProgressIndex(0);
     progressAnim.setValue(0);
     setTimeout(() => setShowMnemonic(true), 200);
-    AsyncStorage.setItem('sentinel_mnemonic_part_local', parts[0].join(' ')).catch(() => {
+    AsyncStorage.setItem(vaultKeys.MNEMONIC_PART_LOCAL, parts[0].join(' ')).catch(() => {
       // Best-effort local store; UI remains available
     });
   };